1. Technical Field
The invention relates to a toner for electrostatic charge development which is used for developing an electrostatic latent image formed by electrophotography, electrostatic recording method, or the like with a developer, and a method for manufacturing the toner, and an image forming method using this toner for electrostatic charge development.
2. Related Art
A method of visualizing image information via an electrostatic charged image by, for example, electrophotography has been used in various fields in recent years. An image is visualized in the electrophotographic method through the steps of forming an electrostatic latent image on a photoreceptor by charging and exposing the photoreceptor, developing the electrostatic latent image with a developer containing a toner, and transferring and fixing the image.
As a developer used here, two-component developers including a toner and a carrier, and one-component developers using only a magnetic toner or a nonmagnetic toner are known. For the manufacturing method of the toner, a kneading pulverization method in which a thermoplastic resin is melted and kneaded with a pigment, a charge control agent, and releasing agent such as wax, cooled and then pulverized and classified is generally used. To these toners, inorganic or organic particles may be added onto the surfaces of the toner particles, if necessary, in order to improve fluidity and clean properties. While this method can provide a quite excellent toner, it involves some problems as described below.
The shape and surface structure of a toner manufactured by the conventional kneading pulverization method becomes irregular. In the kneading pulverization method, it is difficult to intentionally control toner shape and toner surface structure, although they subtly vary depending on the pulverizability of materials to be used and the conditions of pulverizing. Moreover, the range of selection of the materials is limited in the kneading pulverization method. Specifically, in the kneading pulverization method, a dispersion of a resin and a colorant must be a materials which is sufficiently fragile and can be finely-divided with an economically practical production apparatus. However, finer powders may be undesirably generated by a mechanical shear force applied to a toner in a developing apparatus or the shape of the toner may be changed, when the resin colorant is made to more fragile to satisfy the above requirements.
Because of these influences, in one-component developers, it becomes easy by the expansion of the particle size distribution to cause toner scattering at the time of development, and to induce deterioration in image by a decrease in developability due to change in the toner shape. When a toner is prepared by internally adding a large amount of releasing agent such as wax, exposure of the releasing agent on the surface is caused in the toner in some cases, depending on the combination of releasing agent and thermoplastic resin. In particular, in a combination of a thermoplastic resin which has increased elasticity due to a high molecular component and is slightly difficult to pulverize, and a fragile wax which is fragile such as polyethylene and polypropylene, exposure of these wax components are observed on the toner surface in many cases. Exposure of the wax components is advantageous in the removability at fusion and cleaning of untransfered toner from a photoreceptor. However, polyethylene in a superficial layer is easily transfered by mechanical force, and the developing roll, the photoreceptor and the carrier are liable to be contaminated, and thus reliability may be decrease.
Further, when a toner contains magnetic particles, the resistance to bending of the fixed image is low, there is a limitation to the processing speed in order to avoid this problem, and the application of the toner to high-speed processing is difficult.
In recent years, methods for manufacturing a toner by a emulsification-aggregation method has been proposed as a means for enabling intentional control of the toner shape and the surface structure. Generally This is a manufacturing method such that after a resin dispersion solution is prepared by emulsion polymerization or the like, and further, a colorant dispersion solution in which the colorant is dispersed in a solvent is prepared, these dispersion solutions are mixed to form an aggregate corresponding to the toner particle diameter, and the aggregate is fused and coalesced by heating to make a toner.
The shape may be controlled to some extent by this method, and the charging characteristics and durability of the toner may be improved. However since the inner structure of the toner becomes almost unipaper Problems have been left in the removability of a recording media on which an image is formed from a fusing roll, dependency of charging stability on the environmental conditions, and the like.
In such electrophotographic processes, in order to stably maintain toner performance even under various mechanical stresses, it is necessary to suppress exposure of the releasing agent on the surface, enhance the surface hardness without deteriorating the fixability and, at the same time, improve the mechanical strength of the toner itself, and satisfy the need for both sufficient charging property and fixability.
In more recent years, as demand for high image quality rises in the formation of images, there has been a significant trend to make the toner diameter smaller to obtain images of high definition. However, when the toner is simply made small while the particle size distribution is left unchanged, on the minute powder side, the problems of contamination of the carriers and photo receptor and scattering of the toner become significant due to toner particles having an extremely small diameter and it is difficult to achieve the high image quality and the high reliability at the same time. Therefore the particle diameter distribution should be made sharp and the particle diameter is reduced.
Moreover, uniform charging characteristics, stability, toner strength, and sharpness of the particle size distribution are becoming more and more important from the viewpoint of increasing speeds and producing accompanying low energy consumption in recent years. In addition, when increasing speeds energy saving, and the like of these apparatuses, further low-temperature fixability is also required. From these points, toners having excellent properties, manufactured by wet manufacturing methods suitable for manufacturing a toner having sharp particle size distribution and small particle diameter, include toners coalesced by aggregating and fusing, toners produced by suspension polymerization, toners produced by the suspension granulation, and toners coalesced by emulsion suspension aggregation.
In general, the component of releasing agent has an internally added polyolefin wax in order to prevent low-temperature offsetting in the fusing process. Moreover, accompanying this, a small amount of silicone oil is uniformly applied to the fusing roller, which is aimed at improving offsetting at high temperatures. Therefore, it is preferred as silicone oil adheres onto the surface of recording media on which an image is formed and it is not unpleasant to handle because of sticky feeling. For this reason, a toner for oilless fusing which contains a large amount of the releasing agent component has been proposed.
Addition of a large amount of releasing agent can improve the removability to an extent. However, it is difficult to obtain stable removability since a problem of compatibility between the binder component and releasing agent occurs, and stable exudation of the releasing agent is not uniform. Further, since the means for controlling the aggregating force of a binder resin in a toner depends on the Mw and Tg of the binder, it is difficult to directly control the stringiness and the aggregating property of the toner at fusion. Further, liberated components from the releasing agent may cause inhibition of charging.
As a method for resolving these problems, a method of enhansing the rigidity of binder resin by adding a high molecular weight component and a method of improving the removability in oilless fusing by rigidity compensation by the introduction of chemical crosslinking structures into the binder resin and consequentially decreasing the stringiness of toner at a fusing temperature have been proposed.
Moreover, when simply a component of a crosslinking agent is added in the binder, because the viscosity of the toner, that is, aggregation force at the time of fusing becomes large and the rigidity of the binder resin itself is increased, the temperature dependency and the dependency on the applied amount of the toner in the oilless removing are improved to some extent. However, it is difficult to obtain simultaneously surface glossiness of fixed images and the bending resistance of fixed images also becomes low.
In addition, though the molecular weight between intertwining points is certainly increased and the flexibility of the fixed image itself is somewhat improved by simply making the molecular weight of the crosslinking agent high, it is difficult to obtain an appropriate balance between elasticity and viscosity. As a result, the temperature dependency of release in oilless fusing and the dependency on the applied amount of the toner for surface glossiness of the fixed image causes difficulties. Moreover, in the one-component developers which contain metal particles in the toner, this tendency becomes even more significant. Particularly, when the toner is used in energy saving-type fusing apparatus with low temperature and low pressure, and the copier and printers of fast printing type, it is basically difficult to obtain a satisfactory fixed images.
In a one-component developer using magnetic metal particles as a colorant, since the specific gravity of the toner can definitely be increased in a fusing kneading pulverization method, which is a dry process, the coloring function and the charging function can be suitably controlled. Further, stable charging property and coloring property can be manifested at the same time, the system for controlling the toner concentration in the electrophotographic process can be simplified, and an extremely useful toner can be obtained. However, since the controllability of a structure such as a core/shell structure of a toner is deteriorated, there is a problem in fluidity, and it is difficult to obtain a precise image.
In order to solve these problems, new toners and processes are provided, such as an emulsion aggregation and coalescent method (heterogeneous aggregating method), a suspension polymerization method, a solubility suspension granulation method, and a solubility emulsion aggregating coalescent method which are wet processes. However, since these wet processes produce toner particles in an acidic or alkaline aqueous solution, when the fine magnetic metal particles are dispersed in these solution, the surface property of the magnetic material itself is greatly changed by oxidation or reduction and, under acidic conditions, the surface of the magnetic material oxidizes, the color changes to a reddish-brown color and, under the alkaline conditions, iron hydroxide particles are produced, and a change in the magnetism occurs and, therefore, the charging property of the toner is suppressed
In addition, under the acidic conditions, a dissolved magnetic particle ions are present in an aqueous solution and, in emulsion aggregation and coalescent methods, since the ion balance in an aggregation system breaks down, it becomes difficult to control the aggregation rate; in a suspension polymerization system, since polymerization is suppressed, it is particularly difficult to control the particle diameter. Further, in a solubility suspension granulation method and a solubility emulsion aggregating coalescent method, it is difficult to obtain particle stability upon granulation or emulsification.
A method of improving these problems has been proposed where the solubility of magnetic metal particles to acid and alkali is decreased in advance, as a means of solving these problems. Problems of manufacturability and color of the toner are certainly improved by these measures.
However, in energy-saving fixing systems of recent years and improved productivity of increased printing speeds, as described above, it is difficult to avoid the lowering of the fusing properties, that is, the bending resistance of the fixed image.